Image sensing device with non-linear converting means

ABSTRACT

An image sensing device is provided with a switching circuit for a selection between a field reading mode and a frame reading mode in reading signals formed at picture elements of an image sensor; and an overflow control circuit for controlling and changing the overflow level of each of the picture elements from one level over to another in response to the selecting action of the switching circuit. The device is arranged to store, at an EEPROM, data relative to the overflow levels of each of the picture elements differently set for the field and frame reading modes.

This is a continuation application under 37 CFR 1.62 of priorapplication Ser. No. 554,746, filed Jul. 18, 1990, abandoned, which is adivision of application Ser. No. 453,213, filed Dec. 13, 1989, patentedOct. 16, 1990, U.S. Pat. No. 4,963,980, which is a continuation ofapplication Ser. No. 151,573, filed Feb. 2, 1988, abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an image sensing device which is arranged topermit selection of either a field reading mode (hereinafter referred toas the field mode) or a frame reading mode (hereinafter referred to asthe frame mode).

2. Description of the Related Art

There are electronic still cameras of the kind arranged to permit aselection between a field image signal and a frame image signal informing an image signal within an image sensor before reading it out forrecording. An example of the camera of this kind is disclosed inJapanese Laid-Open Patent Application No. SHO 58-48455.

FIG. 6 of the accompanying drawings shows the switching circuit of anelectronic still camera of the above stated kind arranged to permit aselection between the field mode and the frame mode. The illustrationincludes an adder 60; signals 62₁ and 62₂ produced from odd-number fieldpicture elements (hereinafter referred to as ODD picture elements) andeven-number field picture elements (hereinafter referred to as EVENpicture elements) respectively; change-over switches S1 and S2 which areprovided for a selection between the field mode and the frame mode;field mode selecting contacts A1 and B1; frame mode selecting contactsA2 and B2; a terminal A3 which is provided for reading a field image;and terminals B3 and B4 which are provided for reading the odd- andeven-number field portions of the frame image signal, respectively. Whenthe moving contacts of the field mode/frame mode change-over switches S1and S2 are on the side of the frame mode selecting contacts A2 and B2,the information of the ODD picture elements and that of the EVEN pictureelements are serially read out from the reading terminals B3 and B4 forevery field. In case that the moving contacts of the change-overswitches S1 and S2 are on the side of the field mode selecting contactsA1 and B1, the information of the ODD picture elements and that of theEVEN picture elements are added together by the adder 60 and read outfrom the reading terminal A3 as a field image.

FIG. 7(A) shows relations obtained between the quantity of incidentlight and the intensity of an output signals obtained in the field modeand in the frame mode. FIG. 7(B) shows a relation between theilluminance of a object and the intensity of the output signal obtainedin the frame mode with the incident light quantity optically reduced to1/2 by means of an ND filter or an exposure device. Referring to thesedrawings, reference symbols Vs1 and Vs2 denote saturation outputvoltages obtained in the frame mode and in the field mode. A symbol Hsdenotes saturation light quantity on the image sensing plane. SymbolsLs1 and Ls2 denote saturation degrees of object's illuminance obtainedin the frame mode and in the field mode respectively. As apparent fromthe characteristic curves of FIG. 7(A), the sensitivity obtained on theimage sensing plane is higher by two times in the field mode than in theframe mode.

The electronic still camera is generally arranged to perform a processof suppressing the high luminance portion of an image signal forobtaining an effect called a knee effect (hereinafter referred to as aknee process) with regard to the problem relative to the dynamic rangesof the image sensor and signal processing system or the visual sensationcharacteristic of human. FIGS. 8(A), 8(B), 9(A) and 9(B) respectivelyshow the result of the knee process applied to the characteristics shownin FIGS. 7(A) and 7(B). In these drawings, a reference symbol Vk denotesa knee point voltage which is the start voltage of the knee process; anda symbol Vmax denotes a maximum signal level determined by the dynamicrange of the signal processing system. Further, FIGS. 8(A) and 9(A) showthe results of the knee process carried out in such a way as to causethe saturation voltage obtained in the frame mode to become the abovestated maximum signal level Vmax. FIGS. 8(B) and 9(B) show the result ofthe knee process carried out to cause the saturation voltage obtained inthe field mode to become the voltage Vmax.

As apparent from these drawings, the output signal intensity obtained inthe field mode with the knee process carried out to bring the saturationvoltage of the frame mode to the voltage Vmax comes to exceed thedynamic range of the signal processing system. Whereas, in case that theknee process is carried out to bring the saturation voltage obtained inthe field mode to the voltage Vmax, the output signal intensity does notreach the voltage Vmax. In the latter case, therefore, there takes placesome contrast blunting effect.

Further, in the case of the conventional image sensing device arrangedto obtain the sum of the electric charge values of two picture elementswithin the sensor, the problem of blooming has occurred when the sumcomes to exceed the saturation potential of the V-CCD thereof.

SUMMARY OF THE INVENTION

This invention is directed to the solution of these problems of theprior art. It is, therefore, the principal object of this invention toprovide an image sensing device which has the optimum dynamic range andis capable of giving adequate pictures both in the field and framemodes.

It is another object of this invention to provide an image sensingdevice which is capable of preventing occurrence of blooming even iffield reading is performed with a transfer CCD which has a smallsaturation potential and is designed for frame reading.

To attain this object, an image sensing device arranged according tothis invention to permit selection between the field mode and the framemode is provided with knee effect change-over means for changing theknee effect on the output signal of the image sensor from one value overto another in such a way as to give images with the optimum dynamicrange both in the field mode and in the frame mode.

Further, to attain the above stated object, an image sensing devicearranged as another embodiment of the invention is provided withoverflow level change-over means for changing one overflow level over toanother level for each of picture elements in response to a selectionmade between field reading and frame reading. With a transfer CCD whichhas a small saturation potential employed as the image sensor, the abovestated arrangement of the embodiment effectively prevents occurrence ofblooming because the saturation potential of the transfer CCD is neverexceeded by virtue of the overflow control even in the event of fieldreading.

Other objects and features of this invention will become apparent fromthe following detailed description of embodiments thereof taken inconnection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are block diagrams showing electronic still cameras asfirst and second embodiments of this invention.

FIGS. 3(A) and 3(B) show the output signal intensity of the electronicstill camera of this invention obtained in relation to the quantity oflight incident on the camera and to the illuminance of an object withthe knee point changed from one level over to another according to aselection made between the field mode and the frame mode.

FIGS. 4(A) and 4(B) likewise show the output signal intensity inrelation to the quantity of light incident on the still camera and toobject illuminance with a knee compression rate changed according to aselection made between the field and frame modes.

FIGS. 5(A) and 5(B) likewise show the output signal intensity inrelation to incident light quantity and to object illuminance obtainedwith change-over of the level of knee point and the knee compressionrate.

FIG. 6 is a diagram showing a switching circuit of the conventionalelectronic still camera which is arranged to permit a selection betweenthe field mode and the frame mode.

FIGS. 7(A) and 7(B) respectively show the output signal intensity of thecamera in relation to the quantity of incident light and to theilluminance of an object obtained in the field and frame modes.

FIGS. 8(A) and 8(B) show the output signal intensity of the conventionalcamera obtained in relation to incident light quantity on the basis ofsensitivity obtained in the frame and field mode.

FIGS. 9(A) and 9(B) show the output signal intensity of the conventionalcamera obtained in relation to the illuminance of an object on the basisof sensitivity obtained in the frame and field modes with the incidentlight quantity changed according to a selection made between the framemode and the field mode.

FIG. 10 is a block diagram showing in outline the arrangement of theessential parts of a third embodiment of this invention.

FIG. 11(A) shows in outline a CCD employed in the third embodiment.

FIG. 11(B) shows the distribution of potential in the direction of depthof a vertical overflow drain type photo diode which is popularly in use.

FIG. 11(C) shows the sensitivity characteristic of a CCD used for thethird embodiment.

FIG. 12 is a driving timing chart showing a filed reading operation.

FIG. 13 is a driving timing chart showing a frame reading operation.

FIGS. 14(A) and 14(B) show in outline the image sensing sequence ofprocesses of an electronic still camera which is arranged as a fourthembodiment of this invention.

FIG. 15 shows the sensitivity characteristic of an ordinary CCD.

FIG. 16 is a block diagram showing a fifth embodiment of the inventionwhich has a memory arranged within the system control part of the thirdembodiment arranged as shown in FIG. 10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Examples of embodiment of this invention are described below withreference to the accompanying drawings:

FIG. 1 shows in a block diagram a first embodiment of this invention. Inthis case, the invention is applied to an electronic still camera whichis selected by way of example as an image sensing device. Referring toFIG. 1, an image sensing optical system 1 consists of a lens, adiaphragm, a shutter, etc. A CCD (charge coupled device) 2 is employedas an image sensor. A preamplifier 3 is arranged to amplify the outputof the CCD 2. A reference numeral 4 denotes a knee process circuit. Anoutput amplifier 5 is arranged to amplify the output of the knee processcircuit 4. An image recorder 6 is a magnetic disc type recorder or thelike. A numeral 7 denotes a field mode/frame mode selection circuit. Adrive circuit 8 is arranged to drive the CCD 2. An AE circuit 9 isprovided for exposure control.

The electronic still camera which is arranged in this manner as thefirst embodiment of this invention operates in the following manner: Theimage sensing optical system 1 obtains an image of an object to bephotographed. The image is formed on the CCD 2 and is then undergoes aknee process performed through the preamplifier 3 by the knee processcircuit 4. The knee process circuit 4 thus produces an image signal,which is recorded via the output amplifier 5 by the image recorder 6.

The position of each of the AE circuit 9 and the drive circuit 8 isswitched from one position over to another according to the switchingposition of the field mode/frame mode selection circuit 7. In the fieldmode, the AE circuit 9 determines an exposure condition suited forsensitivity obtained in the field mode. An object's image obtained underthis exposure condition is photo-electric converted by the CCD 2. Anelectric charge thus obtained is read out and produced in accordancewith a method determined by the drive circuit 8. More specifically, anODD picture element portion and an EVEN picture element portion of asignal are added together within a vertical register, a horizontalregister or a CCD output amplifier or the like and are produced as anoutput of the CCD 2. Further, at the knee process circuit 4, a kneeprocess characteristic for the field mode is selected in accordance witha signal from the selection circuit 7 and a knee process is appositelycarried out.

In the frame mode, the position of the selection circuit 7 is shifted toa frame mode selecting position. Then, the AE circuit 9 determines anexposure condition suited for the sensitivity obtainable in the framemode. The CCD 2 performs the photo-electric converting action on anobject's image obtained under the exposure condition, which allows anexposure to be made about twice as much as an exposure to be allowed inthe field mode. Electric charge values of the ODD and EVEN pictureelements which are cumulatively stored at the CCD 2 are serially readout in accordance with the signal output of the drive circuit 8. Theoutput of the CCD 2 is supplied via the preamplifier 3 to the kneeprocess circuit 4. The circuit 4 selects a knee process characteristicfor the frame mode in accordance with a signal produced from theselection circuit 7 and appositely performs a knee process. A signalthus processed is recorded by the recorder 6.

The electronic still camera of this embodiment is arranged to change theincident light quantity from one quantity over to another according tothe selection of the field mode or the frame mode. Therefore, the outputcharacteristic of the amplifier 3 becomes as shown in FIG. 7(B). In thecase of another embodiment of this invention in which the AE circuit 9is not change-over controlled according to the selection of the fieldmode or the frame mode, the output characteristic of the amplifier 3becomes as shown in FIG. 7(A). FIGS. 3(A) and 3(B) show examples of kneeeffect change-over, which correspond to what is shown in FIGS. 7(A) and7(B). Referring to FIGS. 3(A) and 3(B), a reference symbol Vk1 denotes aknee point voltage obtained in the frame mode and a symbol Vk2 a kneepoint voltage in the field mode.

Both the examples are capable of having the saturation level of thepicture element output coincide with the upper limit Vmax of the dynamicrange of the signal processing system even in a high luminance region inboth the field mode and the frame mode by virtue of the arrangement toshift the knee point voltage to the voltage Vk2 or Vk1 according to theselection of the field or frame mode as shown in these drawings. Animage signal, therefore, can be obtained in an adequate state. Further,these examples have the same knee compression rate (the inclination ofthe characteristics shown).

FIGS. 4(A), 4(B), 5(A) and 5(B) show other examples of embodimentarranged to make knee effect change-over in different manners. In thecases of FIGS. 4(A) and 4(B), the knee point voltage is arranged to bethe same both in the field and frame modes while the knee compressionrate is arranged to vary according to the selection of the field mode orthe frame mode. This arrangement also gives an adequate image. In thecase of FIGS. 5(A) and 5(B), both the knee point voltage and the kneecompression rate are arranged to be variable. This arrangement gives abetter image.

In the case of the first embodiment shown in FIG. 1, the knee processcircuit 4 is thus arranged to have the knee effect change-overcontrolled in a manner as shown in FIG. 3(B), 4(B) or 5(B). In case thatthe AE characteristic is not changed according to a selection of thefield mode or the frame mode, the knee effect change-over is controlledin the manner as shown in FIG. 3(A), 4(A) or 5(A).

FIG. 2 is a block diagram showing an electronic still camera which isarranged according to this invention as a second embodiment thereof. Theillustration includes an adder 20; an image sensing optical system 21; aCCD 22; a knee process circuit 24A for the field mode; knee processcircuits 24B1 and 24B2 for an ODD picture element signal and for an EVENpicture element signal obtained in the frame mode; image recorders 26₁and 26₂ ; a field mode/frame mode selection circuit 27; a CCD drivecircuit 28; an AE circuit 29; field mode/frame mode selection switchesS1 to S4; field mode selection contacts A1 to A3; and frame modeselection contacts B1 to B4.

In the field mode of the second embodiment, the image information valuesof two fields are added together not within the CCD 22 but by means ofan adder 20 which is disposed outside of the CCD 22. Further, in thefield mode, the moving contacts of the selection switches S1, S2 and S3are shifted by the selection circuit 27 to the field mode selectioncontacts A1, A2 and A3 while the selection switch S4 is shifted to anopen state. In the frame mode, the selection circuit 27 shifts thepositions of the selection switches S1 to S3 to the frame mode selectioncontacts B1 to B3 and closes the selection switch S4. In this manner, aplurality of knee process circuits are used instead of the use of asingle knee process circuit. The knee process is carried out byselectively using these plurality of circuits. With the exception ofthis, the rest of the arrangement of the second embodiment is similar tothe first embodiment.

As apparent from the foregoing description, the first and secondembodiments of this invention are capable of giving an adequate imagewith the optimum dynamic range by changing the knee processing operationon the output signal of the image sensing device according to theselection of the field mode or the frame mode.

FIG. 10 shows in a block diagram an image sensing device which isarranged as a third embodiment of this invention to permit a selectionbetween the field reading mode and the frame reading mode. Theillustration includes a CCD 31; a signal processing circuit 32; arecorder RD; a drive circuit 33 which is arranged to drive the CCD 31; aclock signal generating circuit 34 which is arranged to generate pulsesfor driving the CCD 31 and pulses to be used by the signal processingcircuit 32; a bias circuit 35 which is arranged to generate a biasvoltage necessary for the CCD 31; a selection switch 36 which isarranged to change the field reading mode over to the frame reading modeand vice versa; a system control part 37 which is arranged to controlthe whole system; a lens 38 with a diaphragm; and a shutter 39. Thesystem control part 37 causes the bias circuit 35 to change an overflowdrain voltage which will be described later between a value Va andanother value Vb according to a selection made between the field modeand the frame mode.

FIG. 11(A) shows in outline the arrangement of the CCD employed in thethird embodiment shown in FIG. 10. FIG. 11(B) shows the distribution ofpotential of the CCD in the direction of depth of a photo diode part.The CCD is an inter-line type CCD, wherein the photo diode part whichserves as picture elements having a vertical overflow drain arrangement.These illustrations include photo diodes 40; a vertical CCD register 41(hereinafter referred to as V-CCD); a horizontal CCD register 42(hereinafter referred to as H-CCD); an electric charge detector 43;transistor switches 44; terminals φv1, φv2, φv3 and φv4; and the abovestated overflow drain voltage VOFD 45.

With an object's image of a high degree of luminance formed on the imageplane, when the potential of the photo diode in the picture element areahaving the high luminance extremely rises, an excess electric chargegenerated at the photo diode 40, in this case, overflows a potentialbarrier determined by the overflow drain voltage VOFD 45 and is thusproduced toward a substrate. Blooming is prevented by this arrangement.

Further, as shown in FIG. 11(B), there is a crest of potential at adepth point dp. Assuming that the overflow drain voltage VOFD is set ata voltage Va, the potential crest is at a voltage Pa. With the overflowdrain voltage VOFD assumed to be set at a voltage Vb, the potentialcrest becomes a voltage Pb. Therefore, the saturation potential of thephoto diode decreases accordingly as the crest of potential is loweredby changing the overflow drain voltage VOFD. Conversely, the saturationpotential of the photo diode becomes higher accordingly as the crest ofpotential is made to become higher. Further the voltages Va and Vb arein the relation of Va>Vb.

In the case of the third embodiment, the saturation potential of thephoto diode is controlled by controlling the overflow drain voltageVOFD. In other words, the dynamic range is increased by setting theoverflow drain voltage VOFD at a relatively low level in the frame mode.In the field mode, the blooming preventing effect is increased bysetting the overflow drain voltage VOFD at a relatively high level.

FIG. 11(C) shows in a graph the sensitivity characteristic of the CCD ofthe third embodiment shown in FIG. 10. The axis of abscissa of the graphindicates the quantity of light and the axis of ordinate the level ofsignal potential. The bias circuit 35 is controlled to set the overflowdrain voltage VOFD at a higher level in the field mode than in the framemode as shown in FIG. 11(C). In other words, possible blooming in thefield mode can be prevented without impairing the dynamic range for theframe mode by arranging the sum of the electric charges of the photodiodes of rows A and B not to exceed the saturation potential of theV-CCD.

FIG. 12 shows in a timing chart a driving action performed in the fieldmode on the CCD arrangement shown in FIG. 11(A). FIG. 13 is anothertiming chart showing a driving action on the CCD arrangement asperformed in the frame mode.

Referring to the block diagram of FIG. 10, the electric charge is readout from the CCD 31 according to the timing of FIG. 12 when the positionof the switch 36 is shifted to a field mode selecting side. Referring toFIG. 11(A), the terminals φv1 and φv3 are arranged to serve combinedfunctions as transfer electrodes for transferring the electric chargefrom within the V-CCD 41 and also as electric charge shift gates forshifting the electric charge from the photo diodes PD disposed forpicture elements to the V-CCD 41. As shown in FIGS. 12 and 13, theterminals φv1 and φv3 are arranged to have three levels. The output ofthe V-CCD 41 is transferred when clocking is made at a point between alow (L) level and a medium (M) level of these terminals. When the valueof the terminals becomes a high (H) level, the electric charge isshifted from the photo diode 40 to the V-CCD 41 to be transferred to theH-CCD 42 at the timing of ensuing horizontal blanking for forming animage signal. The reading operations in the field and frame modes of theembodiment are as described below:

Referring to FIG. 12, in reading for an odd-number field, a pulseobtained at a part (A) causes the levels of the terminals φv1 and φv3 tobecome high levels at the same time. This causes the electric charges ofthe rows A and B of the photo diode 40 to be simultaneously shifted tothe V-CCD 41. The electric charges of the rows A and B are caused to beadded together by a pulse obtained at a part (B) and the electric chargethus obtained is transferred into the V-CCD 41. Then, the electriccharges of other rows A' and B' are likewise added together andtransferred. Next, in reading for an even-number field, the electriccharges of the rows A and B are caused to be simultaneously read out bya pulse obtained at a part (C). The reading point of time is the same asin the case of reading for the odd-number field. However, since theelectrode (or terminal) φv1 is at a medium level at a part (D) in thisinstance, the charge of the row A is not transferred in response to thepulse of the part (D). Meanwhile the electric charge of the row B andthat of the row A', i.e. an interlaced row of the odd-number field, areadded together. In accordance with this method of reading, the signal ofeach photo diode 40 is normally read out in one field period. Therefore,this method is called field reading. A reference symbol CBL denotes ablanking pulse.

Next, the frame reading operation which is as shown in FIG. 13 is asfollows: In reading for an odd-number field, a pulse obtained at a part(E) causes the level of the terminal φv1 to become high (or H) to allowthe electric charges of the rows A and A' to be read out. In reading foran even-number field, a pulse obtained at a part (F) causes the level ofthe terminal φv3 to become high to allow the electric charges of rows Band B' to be read out. In this method of reading, the signal of eachphoto diode 40 is normally read out in one frame period, i.e. in twofield periods, and is thus called frame reading.

FIGS. 14(A) and 14(B) schematically show the image sensing sequence ofprocesses of an electronic still camera which is arranged as a fourthembodiment of this invention. The field reading and the frame reading ofthis camera are as follows: Referring to FIG. 14(A) which shows thefield reading, a dark current within the CCD 31 is cleared within agiven period of time in response to a shutter release signal. Then, anexposure is effected when the shutter 39 is opened. Then, the exposureis terminated by closing the shutter 39. After this, the electric chargeof the photo diodes 40 of the row A and that of the photo diodes 40 ofthe row B are simultaneously read out. The electric charge of the row Aand that of the row B are added together before they are read out as asignal of an odd-number field. The details of the timing for reading arethe same as the timing for the odd-number field shown in FIG. 12.

In the case of frame reading which is as shown in FIG. 14(B), theprocesses of the operation of the camera up to an exposure is the sameas in the case of field reading as shown in FIG. 14(A). In reading theelectric charge, however, the charge of the photo diodes 40 of the row Ais read out for an odd-number field while that of the row B is read outfor an even-number field. The details of timing for reading are the sameas those shown in FIG. 13.

FIG. 15 shows the sensitivity characteristic of the CCD 31. The axis ofabscissa of FIG. 15 indicates light quantity and the axis of ordinatethe potential level of the signal. A reference symbol VSAT denotes asaturation potential. A symbol VSAT-V-CCD denotes the saturationpotential of the V-CCD. Symbols L1 and LSAT respectively denote a lightquantity at which the V-CCD and photo diodes reach their saturationpotentials. A symbol C1 denotes an output curve of the photo diodes. Asymbol C2 denotes a curve representing the result of addition of twopicture-element portions of the signal.

With the degree of exposure assumed to be the same, the field readingmode gives a sensitivity or signal level which is twice as high as thelevel obtained in the frame reading mode as shown in FIG. 15. However,since the number of picture elements of the signal obtained by framereading is twice as much as the number of picture elements obtainable byfield reading, the resolution of frame reading is twice as high as thefield reading. Generally, in the case of a CCD which is designed forframe reading, the V-CCD is arranged to be as narrow as possible for thepurpose of increasing the sensitivity and the opening rate of the photodiodes is arranged to be as large as possible for an increased lightreceiving area. In that instance, it is generally impossible to arrangethe V-CCD to have a large saturation potential capacity. Generally,therefore, the saturation potential of the V-CCD is about 1.5 times ashigh as that of the photo diode as shown in FIG. 15. Therefore, if thedevice of FIG. 11(A) is used for field reading as it is, the sum ofelectric charges of the photo diodes of rows A and B would exceed thesaturation potential of the V-CCD to cause blooming as two pictureelements adjacently arranged in the vertical direction are receiving thelight quantity L1.

Whereas, in accordance with this invention, this problem is solves bychanging the overflow bias level according to the selection of the fieldreading mode or the frame reading mode.

FIG. 16 shows in a block diagram a fifth embodiment of this invention.In this case, a memory arrangement is provided within the system controlpart 37 of the embodiment shown in FIG. 10. The system control part 37in this case includes an EEPROM 46. The potentials Va and Vb of theoverflow drain voltage VOFD is stored by this EEPROM 46. Thisinformation is arranged to be supplied via a digital-to-analog (D/A)converter 47 to the bias circuit 35. The fifth embodiment which isarranged as shown in FIG. 16 operates as follows: The optimum values Vaand Vb of the overflow drain voltage VOFD for the field reading andframe reading modes are written beforehand into the EEPROM 46. When thefield reading mode or the frame reading mode is selected by the switch36, the value of the overflow drain voltage VOFD which is apposite tothe reading mode selected is read out from the EEPROM 46. The valueinformation thus read out is converted into a voltage by the D/Aconverter 47 and is applied via the bias circuit 35 to the CCD 31.

This arrangement not only enables the embodiment to supply the CCD 31always with the optimum overflow drain voltage VOFD irrespectively ofthe selection of the field or frame reading mode but also permitsadjustment with digital data instead of volume adjustment. Therefore,the number of volumes can be reduced. The arrangement is therefore quiteadvantageous in making an arrangement for automatic adjustment.

While the embodiment described is arranged to control the overflow levelby changing the drain voltage of the vertical overflow drain, thisarrangement may be changed, for example, as follows: In cases whereoverflow is controlled by means of electric charge recoupling pulses asdisclosed, for example, in Japanese Laid-Open Patent Application No. SHO59-153385, the overflow level is lowered by increasing the frequency ofthe pulses and is raised by decreasing the frequency.

As described in the foregoing, the embodiment of this invention isprovided with means for changing overflow level of each picture elementfrom one level over to another. The image sensor, therefore, can bedriven under the optimum condition without impairing the dynamic rangeof the sensor in the case of frame reading and without impairing ablooming resisting characteristic in the case of field reading.

The arrangement to have information on the overflow level written intothe EEPROM and to supply from the EEPROM to the image sensor the optimumoverflow level information in accordance with a selection made betweenthe field reading mode and the frame reading mode obviates the necessityof volume adjustment. The embodiment is, therefore, highly advantageousin respect to automatic adjustment.

What is claimed is:
 1. An image sensing device comprising:a) imagesensing means having photo-electric converting cells arranged in aplurality of rows and columns to generate electrical signals inaccordance with light incident thereon; b) reading means for readingsaid electrical signals of each of said rows one by one in a first modeand said electrical signals of a plurality of rows by adding themtogether in a second mode, said reading means being arranged to permit aselection between said first and second modes; c) signal processingmeans for effecting a predetermined signal processing of the electricalsignals read from said image sensing means, said signal processing meanshaving a predetermined dynamic range; and d) non-linear converting meansfor non-linearly converting levels of said electric signals inputtedinto said signal processing means, said non-linear converting meansbeing arranged to change its non-linear converting characteristic insaid first mode and said second mode, whereby the maximum level of saidelectric signals in each mode is caused to be substantially equal to themaximum value of the dynamic range of said signal processing means. 2.An image sensing device comprising:change-over means for effectingchange-over between a field reading mode and a frame reading mode inreading signals formed at picture elements of an image sensor; andoverflow control means for controlling and changing an overflow level ofsaid picture elements from one level over to another.
 3. A deviceaccording to claim 2, wherein data which is relative to the overflowlevel of each of said picture elements and is determined for each ofsaid field reading and frame reading modes is stored at an EEPROM.
 4. Animage sensing device comprising:a) image sensing means havingphoto-electric converting cells arranged in a plurality of rows andcolumns to generate electrical signals in accordance with light incidentthereon; b) reading means for reading a signal of each of signals ofeach of said rows one by one in a first mode and electrical signals of aplurality of rows by adding them together in a second mode, said readingmeans being arranged to permit a selection between said first and secondmodes; and c) control means having non-linear converting means fornon-linearly converting said electrical signals and for controlling thesaturation level of said electrical signals in accordance with aselection between said first and second modes, said converting meansbeing arranged to non-linearly convert levels of said electrical signalsin said first mode and said second mode.
 5. A device according to claim4, wherein said reading means is arranged to read signals on a pluralityof interlaced liens in the first mode.
 6. A device according to claim 4,wherein said control means is arranged to change the exposure conditionsuch that an exposure amount to the image sensing means is smaller inthe case of the second mode than in the case of the first mode.
 7. Adevice according to claim 1, further including:d) saturation controlmeans for controlling the saturation level of said electrical signals inaccordance with the selection between said first mode and said secondmode.
 8. A device according to claim 7, wherein said saturation controlmeans is arranged to control the saturation levels of said first andsecond modes to be equal to each other.
 9. A device according to claim4, wherein said control means includes:e) a knee processing means forchanging a knee effect on a signal output of said image sensing meanssuch that the same dynamic range is used in both said first and secondmodes.
 10. A device according to claim 9, wherein said knee processmeans is arranged to change said knee effect on said signal output ofsaid image sensing means by changing one knee point level over toanother according to the selection of said second mode or said firstmode.
 11. A device according to claim 9, wherein said knee process meansis arranged to change said knee effect on said signal output of saidimage sensing means by changing one knee compression rate over toanother according to the selection of said field reading mode or saidframe reading mode.
 12. A device according to claim 9, wherein said kneeprocess means is arranged to change said knee effect on said signaloutput of said image sensing means by changing one knee point level overto another and also by changing one knee compression rate over toanother according to the selection of said field reading mode or saidframe reading mode.
 13. An image sensing device, comprising:a) imagesensing means including a plurality of photo-electric convertingelements for photoelectrically converting a light coming from an objectto be photographed; b) reading means for reading said image sensingmeans, said reading means having a first reading mode in which readcontrol is effected to output a signal having a first predeterminedlevel when said image sensing means senses the object under apredetermined illuminance and a second reading mode in which the readcontrol is effected to output a signal having a second predeterminedlevel higher than said first predetermined level when said image sensingmeans senses the object; and c) converting means for non-linearlyconverting an output signal in the first mode and an output signal inthe second mode in different converting characteristic, respectively,said converting characteristics being automatically changed.
 14. Adevice according to claim 13, wherein said non-linear converting meansincludes a non-linear circuit.
 15. A device according to claim 14,wherein said non-linear converting means includes a knee circuit.